1. Field of the Invention
The present invention relates to a method of transmitting a digital signal along a multipath fading transmission line such as a radio transmission in an urban area.
2. Description of the Prior Art
In recent years, digital signal processing techniques have spread to the field of mobile communications so as to meet increasing demands for communication privacy, intelligent communication, and matching with peripheral communication networks. In urban areas where such demands are most intensive, the quality of communication is considerably lowered by multipath propagation due to reflection and diffraction caused by buildings and surrounding geographical features. In digital transmission, when the propagation delay time difference between waves constituting the multipath is increased to such an extent that it is no longer negligible with respect to the length of a time slot, the bit error rate characteristics are highly degraded by waveform distortions and follow-up failures in synchronous systems.
A first conventional digital signal transmission method will be described, by way of example, with reference to FIGS. 37 through 39.
FIG. 37 is a phase transition waveform diagram showing the phase transition of a signal transmitted by the first conventional digital transmission method. Designated at T is a time slot interval which is a minimum unit for transmitting one data symbol. When the data is a 1, a 180.degree. C. phase transition occurs, and when the data is a 0, no phase transition takes place. This modulation technique is called differential coding BPSK (Binary Phase Shift Keying).
Such a transmitted signal can be detected, for example, by differential detection (delay line detection) using a delay line with a delay of one time slot. As a typical example of multipath, propagation consideration will be given as to how a detected output signal behaves in the presence of a two-wave multipath transmission line having a propagation delay time difference .tau. which is not negligible as compared with the time slot interval. A wave arriving earlier will be called a D-wave, and a delayed wave will be called a U-wave.
FIG. 38 is a diagram explaining how the detected output signal behaves when the transmitted signal as shown in FIG. 37 is subjected to the differential detection in the presence of the two-wave multipath propagation. FIG. 38(a) shows a phase transition of the D-wave. A phase transition of the U-wave which arrives with a propagation delay time difference .tau. from the D-wave is illustrated in FIG. 38(b). The detected output at a particular time is the vector inner product of the combined phase of the two waves at that time and the combined phase of the two waves in a preceding time slot. For example, the detected output in a region B in FIG. 38(c) is the vector inner product of the combined phase of the two waves at a time B' and that at a time B.
FIG. 39 shows the combined phase of the D- and U-waves in order to determine the detected output at each of the times A through C. The ratio of the amplitudes of the D- and U-waves is indicated by .rho., and the phase difference between them by .alpha.. For example, the absolute value of the detected output at the time B is the inner product of vectors OB' and OB in FIG. 39, i.e., the square of the line segment OB. Therefore, the detected outputs at the respective times A through C in FIG. 38(c) are given, using the cosine theorem, as follows:
A . . . indefinite PA1 B . . . a.sub.n (1+.rho..sup.2 +2.rho. cos .alpha.) PA1 C . . . indefinite PA1 A . . . indefinite PA1 B . . . a.sub.n (1+.rho..sup.2 +2.rho. cos .alpha.) PA1 C . . . a.sub.n (1+.rho..sup.2 +2a.sub.n .rho. sin .alpha.) PA1 D . . . a.sub.n (1+.rho..sup.2 +2.rho. cos .alpha.) PA1 E . . . indefinite
where a.sub.n (a.sub.n =.+-.1) is a data sequence being transmitted.
In the regions A and C, the detected outputs are indefinite depending on the data values in the preceding and subsequent time slots. After the differential detection, a low-pass filter is normally placed in order to remove harmonics and undesired noise components. Therefore, the waveform of a final detected output signal is as shown by the dotted line in FIG. 38(c), which is produced by filtering the solid-line waveform of FIG. 38(c), and constitutes a portion of an eye pattern. If .rho. is close to 1 and .alpha. is about 180.degree. , the detected output in the region B which is an effective detected output is substantially zero. Thus, the eye is closed, and the bit error rate characteristics are degraded. At this time, since the ineffective detected outputs in the regions A and C are much larger than the effective detected output in the region B, the eye is largely fluctuated in the direction of the time base, making a reproducing clock unable to follow it, and thereby resulting in a greater degration of the bit error rate (See, for example, "Bit Error Rate Characteristics in Rayleigh Fading Having a Propagation Delay Time Difference" by Onoe et al, Papers of Tech. Group on Commun. Syst., IECE JAPAN, CS81-168, 1982, or "Analysis of Instantaneous Bit Errors due to Multiwave Propagation and An Error Generating Mechanism based on A Bit Synchronous System" by Takai et al, Papers of Tech. Group on Commun. Syst., IECE JAPAN, CS83-158, 1984).
In order to reduce the degradation of the eye pattern and the fluctuation of the eye causing the degradation of the bit error rate characteristics, there has been proposed a method such that the phase transition waveform of a transmitted signal is designed so as to produce a plurality of kinds of detected outputs and the problems are improved by a diversity effect produced by combining the plurality of kinds of detected outputs. One example of such a second conventional digital signal transmission method will be described below with reference to FIGS. 40 through 43.
FIG. 40 shows the phase transition of a signal transmitted by the second conventional digital signal transmission method. T indicates one time slot for data. When the data is 1, the phase is rotated twice in one direction, each by 90.degree. in every half time slot. When the data is a 0, the phase is rotated twice in a different direction from the above, each by 90.degree. in every half time slot. This modulation technique is called DSK (Double Shift Keying).
Such a transmitted signal can be detected by a differential detector having a delay line with a delay of a half time slot as shown in FIG. 41. Denoted in FIG. 41 at 4101 is an input terminal, 4102 a multiplier, 4103 a half-time-slot (T/2) delay element, 4104 a low-pass filter, and 4105 an output terminal. Unlike the first conventional example, the carrier is rotated by 90.degree. in phase by the half-time-slot delay element 4103. As with the first conventional example, consideration will be given as to how a detected output signal behaves in the presence of a two-wave multipath transmission line having a propagation delay time difference .tau. which is not negligible as compared with the time slot.
FIG. 42 is a diagram explanating how the detected output signal behaves when the transmitted signal as shown in FIG. 40 is subjected to the differential detection in the presence of the two-wave multipath propagation. FIG. 42(a) shows a phase transition of the D-wave. A phase transition of the U-wave which arrives with the propagation delay time difference .tau. from the D-wave is illustrated in FIG. 42(b). The detected output at a particular time is the vector inner product of the combined phase of the two waves at that time and a phase attained by rotating the combined phase of the two waves in a preceding half time slot by 90.degree.. For example, the detected output in a region B in FIG. 42(c) is the vector inner product of the combined phase of the two waves at a time B and a phase attained by rotating the phase at a time B' by 90.degree..
FIG. 43 illustrates the combined phase of the D- and U-waves in order to determine the detected output at each of the times A through E. The ratio of the amplitudes of the D- and U-waves is indicated by .rho., and the phase difference therebetween by .alpha.. For example, the absolute value of the detected output at the time B is the square of the line segment OB in view of the fact that the vector OB is perpendicular to the vector OB' in FIG. 43. Therefore, the detected outputs at the respective times A through E in FIG. 42(c) are given, using the cosine theorem, as follows:
where a.sub.n (a.sub.n =.+-.1) is a data sequence being transmitted.
In the regions A and E, the detected outputs are indefinite depending on the data values in the preceding and subsequent time slots. Actually, the cut-off frequency of the low-pass filter 4104 is selected to be low enough to prevent intersymbol interference. Therefore, the output signal that has passed through the low-pass filter 4104 is produced by filtering the solid-line waveform of FIG. 42(c), and constitutes a portion of an eye pattern as shown by the dotted line in FIG. 42(c). Since the regions B, D and the region C produce complementary detected outputs as described above, the eye will not be closed. Furthermore, inasmuch as at least one of these effective detected outputs does not become smaller than the ineffective detected output in the region A or E, any fluctuation of the eye in the direction of the time base is reduced, and any degradation of the bit error rate due to a follow-up failure of a reproducing clock is small.
With the second conventional digital signal transmission method, as described above, the bit error rate characteristics are largely improved in a multipath fading transmission line by a kind of diversity effect by combining the mutually different outputs in the regions B, D and the region C, making high-speed digital transmission possible (See, for example, "A Modulation Technique Suffering from Less Timing Fluctuation of Eye Patterns in Multipath Fading" by S. Ariyavisitakul et al, Papers of Tech. Group on Commun. Syst., IECE JAPAN, CS84-67, 1984).
Because the second conventional digital signal transmission method has two phase transitions per time slot, however, it occupies a frequency bandwidth about twice that of the ordinary phase modulation, and hence is poor in frequency utilization efficiency. This method is, in principle, capable of only binary transmission, and cannot reduce the bandwidth through multivalued transmission. The band width could be reduced to a certain extent by reducing the amount of phase transition or smoothing the phase transition more than a stepped pattern, but the bit error rate characteristics would be highly degraded ("A Further Study of Anti-Multipath Modulation Technique DSK--Analysis of Generalized DSK Modulation and Considerations for a Narrow-Band Scheme" by S. Ariyavisitakul et al, Papers of Tech. Group on Commun. Syst., IECE JAPAN, CS85-108, 1985).
Furthermore, according to the second conventional digital signal transmission method, the bit error rate characteristics can no longer be improved in principle when the delay time difference .tau. exceeds 0.5 in terms of .tau./T which is normalized with the time slot interval T. This is because in a region in which .tau./T is 0.5 or more, the regions B and D disappear, and the diversity effect which would be produced by combining the two kinds of detected outputs is no longer obtained.